1. Field of the Invention
The present invention relates to a fuel injection control device of an internal combustion engine, and more particularly to a fuel injection control device which directly injects fuel into the inside of a combustion chamber of an engine while controlling a fuel pressure in a pressure storage chamber to a high-pressure target fuel pressure.
2. Description of the Related Art
Recently, an internal combustion engine which controls a fuel pressure in a pressure storage chamber such that the fuel pressure assumes an optimum high pressure value for a combustion state and directly injects fuel into a combustion chamber has been commercialized and one example of the constitution of a fuel supply system of this type of internal combustion engine is explained in conjunction with FIG. 8.
In FIG. 8, a high-pressure pump 20 is provided for pressurizing the fuel to a high pressure and the high-pressure pump 20 includes a cylinder 21, a plunger 22 which reciprocates in the inside of the cylinder 21, and a pressurizing chamber 23 which is defined and formed by an inner peripheral wall surface of the cylinder 21 and an upper end surface of the plunger 22. A lower end of the plunger 22 is brought into pressure contact with a cam 25 which is formed on a camshaft 24 of the engine, wherein due to the rotation of the cam 25 induced by the rotation of the camshaft 24, the plunger 22 reciprocates in the inside of the cylinder 21 thus changing a volume inside the pressurizing chamber 23.
Further, an inflow passage 30 which is connected to an upstream of the pressurizing chamber 23 is connected with a fuel tank 32 by way of a low pressure pump 31. Here, the low pressure pump 31 sucks and discharges the fuel in the fuel tank 32 and the fuel discharged from the low pressure pump 31 is regulated to a given low pressure value by a low-pressure regulator 33, and, thereafter, the fuel is introduced into the inside of the pressurizing chamber 23 by way of a check valve 34 when the plunger 22 descends in the inside of the cylinder 21.
On the other hand, a supply passage 35 which is connected to a downstream of the pressurizing chamber 23 is connected to a pressure storage chamber 50 by way of a check valve 36, wherein the pressure storage chamber 50 holds the high-pressure fuel discharged from the pressurizing chamber 23 and, at the same time, distributes the fuel into fuel injection valves 51. Further, the check valve 36 is provided for restricting the back flow of the fuel from the pressure storage chamber 50 to the pressurizing chamber 23.
Further, a relief valve 37 which is connected with the pressure storage chamber 50 is a normally-closed valve which is opened at a given valve-opening pressure or more. That is, when the fuel pressure in the inside of the pressure storage chamber 50 is elevated to the above-mentioned valve-opening pressure or more, the relief valve 37 is opened so that the fuel in the inside of the pressure storage chamber 50 is made to return to the fuel tank 32 through a relief passage 38 and hence, the excessive increase of the fuel pressure in the inside of the pressure storage chamber 50 is prevented.
A discharge amount control valve 10 formed on a spill passage 39 which is connected to the pressurizing chamber 23 in common with the supply passage 35 is, for example, a normally-open electromagnetic valve. During a period in which the plunger 22 is moved upwardly in the inside of the cylinder 21, so long as a valve-opening control of the discharge amount control valve 10 is performed, the fuel which is discharged from the pressurizing chamber 23 to the supply passage 35 is made to return from the spill passage 39 to the inflow passage 30 so that the high-pressure fuel is not supplied to the pressure storage chamber 50. Then, after the discharge amount control valve 10 is closed at a given timing during the upward movement of the plunger 22 in the inside of the cylinder 21, the pressurized fuel discharged from the pressurizing chamber 23 to the supply passage 35 is supplied to the pressure storage chamber 50 through the check valve 36.
To an ECU 60 which constitutes an electronic control unit, detection signals from a rotational speed sensor 62 which detects a rotational speed of an engine 40, an accelerator position sensor 64 which detects a step-in amount of an accelerator pedal 63 and the like are inputted. The ECU 60 determines a target fuel pressure PO based on these engine operation information, and performs a feedback control of open/close timing of the discharge amount control valve 10 such that a fuel pressure PR detected by a fuel pressure sensor 61 which detects the fuel pressure in the inside of the pressure storage chamber 50 agrees with the target fuel pressure PO.
Further, the ECU 60 calculates a fuel injection amount which makes an air-fuel ratio detected by an air-fuel ratio sensor 66 arranged on an exhaust pipe assumes a target air-fuel ratio based on an intake air flow rate detected by an air flow sensor 65, an engine rotational speed detected by the rotational speed sensor 62, the fuel pressure in the inside of the pressure storage chamber 50 detected by the fuel pressure sensor 61 and performs a driving control of the fuel injection valves 51.
In the fuel injection control device having the above-mentioned constitution, the change of various state variables when the engine is shifted from a steady state operation to a deceleration operation is explained in conjunction with a timing chart shown in FIG. 7.
In FIG. 7, until a point of time t1, an intake air flow rate qa1 in response to a step-in amount ap (a fixed value) of the accelerator pedal 63 is taken into the inside of a combustion chamber, wherein the fuel injection flow rate qi1 which assumes the target air-fuel ratio is injected from the fuel injection valves 51 based on the intake air flow rate qa1 and the target air-fuel ratio which is preliminarily set in response to the engine operation state so that the state operation is performed with the engine rotational speed maintained at a fixed rotational speed.
Here, a pump discharge flow rate qp1 which is substantially equal to the fuel injection flow rate qi1 is discharged from the high-pressure pump 20 so that the fuel pressure PR (=ph) in the inside of the pressure storage chamber 50 indicated by a solid line agrees with the target fuel pressure PO (=ph) indicated by a chain line.
When the accelerator pedal 63 is released so that the accelerator step-in amount assumes the zero position at the point of time t1, the decrease of the intake air flow rate is started from qa1 and hence, the decrease of the fuel injection flow rate is also started from qi1. As a result, the generated torque of the engine is lowered so that the engine rotational speed is also lowered gradually. At this point of time, the target fuel pressure PO is changed from the set value ph when the accelerator step-in amount is ap to the set value p1 when the accelerator step-in amount assumes the zero position. Accordingly, the relationship between the fuel pressure PR in the inside of the pressure storage chamber 50 indicated by a solid line and the target fuel pressure PO indicated by a chain line becomes fuel pressure PR (=ph)>target fuel pressure PO (=p1) and hence, the discharge amount control valve 10 is controlled such that the discharge flow rate of the high-pressure pump 20 becomes zero. Then, during a period from the point of time t1 at which the accelerator step-in amount is changed to the zero position to a point of time t2 at which a fuel cut control is started, the fuel in the inside of the pressure storage chamber 50 is consumed corresponding to the fuel injection flow rate injected from the fuel injection valve 51 so that the fuel pressure PR (solid line) in the inside of the pressure storage chamber 50 is gradually lowered from ph to pm.
When the engine rotational speed is lowered to a fuel cut start rotational speed nfcin at the point of time p2, the fuel cut control is started so that the fuel injection flow rate is controlled to zero. When the fuel injection flow rate becomes zero, the consumption of the fuel in the inside of the pressure storage chamber 50 is stopped and the fuel pressure PR (solid line) in the inside of the pressure storage chamber 50 is being held at approximately pm during the period from the point of time t2 to a point of time t3 in which the fuel cut control is executed.
When the engine rotational speed is lowered to the fuel cut finish rotational speed nfcout at the point of time t3, the fuel cut control is finished. Then, the fuel injection flow rate qi2 which assumes the target air-fuel ratio is injected from the fuel injection valves 51 again based on the intake air flow rate qa2 at the point of time t3 and the target air-fuel ratio which is preliminarily set corresponding to the engine operation state.
However, at the point of time t3, the fuel pressure PR in the inside of the pressure storage chamber 50 indicated by the solid line is substantially held at pm. Accordingly, the relationship between the fuel pressure PR (=pm) in the inside of the pressure storage chamber 50 indicated by a solid line and the target fuel pressure PO (=p1) indicated by a chain line becomes fuel pressure PR>target fuel pressure PO and hence, the discharge amount control valve 10 is kept controlled such that the discharge flow rate of the high-pressure pump 20 becomes zero.
After the point of time t3, the fuel in the inside of the pressure storage chamber 50 is again consumed in response to the fuel injection flow rate qi2 which is injected again after finishing of the fuel cut so that the fuel pressure PR in the inside of the pressure storage chamber 50 indicated by the solid line is gradually lowered from pm to p1.
Then, the operation reaches a point of time t4 at which the fuel pressure PR (=p1) in the inside of the pressure storage chamber 50 indicated by the solid line and the target fuel pressure PO (=p1) indicated by a chain line agree with each other, the pump discharge flow rate qp2 which is substantially equal to the fuel injection flow rate qi2 is discharged from the high-pressure pump 20 and, after the point of time t4, the control is maintained in a state that the fuel pressure PR indicated by the solid line and the target fuel pressure PO indicated by a chain line agree with each other.
Here, when the engine is at the high temperature due to the operation thereof, the fuel pressure PR in the inside of the pressure storage chamber 50 exhibits the behavior indicated by a broken line in FIG. 7. That is, in a state that the consumption of the fuel in the inside of the pressure storage chamber 50 is stopped due to the fuel cut (period from the point of time t2 to the point of time t3), the fuel which stays in the inside of the pressure storage chamber 50 having a fixed volume receives heat emitted from the engine and the fuel is volumetrically swelled due to the heating whereby the sharp rise of the fuel pressure such as the fuel pressure PR indicated by a broken line is generated.
When the fuel pressure PR in the inside of the pressure storage chamber PR is sharply elevated during the fuel cut and is held at px and then the fuel cut control is finished at the point of time t3, compared to the target fuel pressure PO (=p1) indicated by a chain line, the injection of the fuel is started again while holding the extremely high-value fuel pressure PR (=px). Further, the time necessary for lowering the fuel pressure PR (=px) to the target fuel pressure PO (=p1) is largely prolonged.
In this manner, when the injection of the fuel is restarted with the high fuel pressure value px which is largely different from the target fuel pressure PO, a penetrating force of the injected fuel spray is increased so that the reachable distance of the fuel is increased whereby the fuel adheres to a top surface of the piston or a cylinder wall. Accordingly, the formation of the air-gas mixture optimum to the engine cannot be realized whereby an exhaust gas is deteriorated, or the fuel pressure PR becomes excessively high so that the fuel injection valve 51 cannot be driven with desired response performance, and the engine may be stopped in a worst case.
As one example of a conventional device which overcomes such a drawback, for example, there has been known a device disclosed in JP-A-11-82105 (hereinafter referred to as patent document 1).
In this patent document 1, a following technique is proposed. That is, when the fuel injection amount is sharply decreased as in the case of sharply decelerating the speed of a vehicle from the high speed traveling, when the fuel pressure PR becomes higher than the target fuel pressure PO by a given value or more, the fuel injection valves 51 are driven with the second fuel injection amount Qbd×K(here, 0<K<1)which is smaller than the first fuel injection amount Qbd which corresponds to the fuel injection amount at the time of performing the engine non-load operation thus positively lowering the fuel pressure.
However, although the device described in this patent document 1 is applicable to a diesel engine having a wide combustible air-fuel ratio range and a gasoline engine which performs the stratified combustion operation, in a spark ignition gasoline engine which performs the uniform combustion operation in which fuel of a amount in the vicinity of a theoretical air-fuel ratio to the intake air flow rate is premixed in the combustion chamber, when the fuel is injected with the second fuel injection amount irrelevant to the intake air flow rate, the air-fuel ratio is largely deviated from the theoretical air-fuel ratio so that not only the exhaust gas is deteriorated but also, in a worst case the air-fuel ratio goes beyond the combustible air-fuel ratio range thus giving rise to the possibility of the occurrence of misfire or an engine stop.